A very large number of different forms of swimming pool alarm have been proposed, and a large number of different alarms have actually been marketed. However, to the best of applicant's knowledge, none of these devices has been entirely satisfactory, the major problem being to provide sufficient sensitivity to trigger the alarm whenever it should be triggered without incurring a large number of false alarms. Whilst from the point of view of demonstration, a high sensitivity to water disturbance is impressive, in practical use it is an unmitigated nuisance unless accompanied by some ability to discriminate between disturbances which require investigation, and those which do not, such as wind disturbances, low flying aircraft, and the impact of twigs and other small objects.
Most of the alarms proposed to date have fallen into three main classes, according to the nature of the alarm transducer. In a first class of alarm, exemplified by the alarms of U.S. Pat. Nos. 3,778,803, 3,475,746, 3,683,353 and 3,723,398, the transducer is essentially wave responsive, triggering the alarm in response to surface disturbances of the pool of greater than a predetermined magnitude. There are various ways in which such devices may be falsely triggered, but their principal failing is that ripples quite large enough to trigger the alarm can readily be generated by wind. A second class of alarm discussed at column 1, lines 22-36 and 52-60 of U.S. Pat. No. 3,778,863 and exemplified by U.S. Pat. No. 2,882,915 (McCoy) is triggered by some form of hydrophone. The problem with this type of alarm is that it can be triggered by loud noises external to the pool, such as low flying aircraft.
A third class of alarm has a pressure sensitive transducer exemplified by the alarm of U.S. Pat. No. 2,935,582 which responds to pressure effects in the water. However, this type of alarm again is too readily set off by minor water disturbances if it is to be sensitive enough to trigger for example in response to a small child or domestic pet struggling in the water.
In order to reduce susceptability to false alarms, it has been proposed to integrate the transducer output, as in U.S. Pat. No. 3,969,712 to Butman et al and U.S. Pat. No. 2,942,247 to Lienau et al.
In a commonly assigned copending application of Edward N. Woolley, Ser. No. 579,576, there is disclosed a swimming pool alarm comprising a triggerable alarm device, a transducer head incorporating a high Q resonator element having an ultrasonic resonant frequency and submersible in a swimming pool, and means sensitive to the amplitude of the resonance of the element and operative to trigger the alarm device. Preferably, the resonator is housed in a tuned cavity and a movable freely agitatable element is housed with the cavity to build up the amplitude of resonance in response to sustained water disturbances. This device as described requires a relatively expensive piezoelectric resonator element, and in its preferred form the effectiveness of the agitation element and cavity are difficult to control.